A gas turbine engine includes a compressor section, a combustion section, and a turbine section. Disposed within the turbine section are alternating rows of rotatable blades and static vanes. The stationary vanes, disposed between rows of rotating blades, stabilize and direct the gas flow from one row of rotating blades to the next row. Such gas flow stabilization optimizes the flow through the turbine section, thereby maximizing the amount of work extracted. As hot combustion gases pass through the turbine section, the blades are rotatably driven, turning a shaft which drives the compressor and other auxiliary systems. The higher the gas temperature, the more energy which can be extracted in the turbine section, and the greater the overall efficiency. In order to increase the turbine section operating temperature capability, nickel-base superalloy materials are commonly used to produce the turbine airfoil blades and vanes, since such materials retain mechanical properties at elevated temperatures.
The stator, or stationary vane, assembly normally mounts in the engine case. While rows of rotor blades extend outwardly from the rotor across the gas flow path, in both the turbine and compressor sections, an array of stator vanes extends inwardly from the engine case across the gas flow path at the downstream end of most blade rows. Such vanes are frequently held in place at the engine case by feet, or lugs, which are engaged by flanges or retaining rings extending inwardly from the outer case. Frequently such lugs are damaged, mis-machined during manufacture, or broken during installation or use. Vanes must operate close to their temperature limits, and if they should be subjected to even brief exposure to higher temperatures, the strength and fatigue capability of the material used may be decreased. Turbine vanes may also undergo distortion, stretching, and elongation during service. This condition, referred to as metal creep, may reduce service life, and in combination with reduced strength and fatigue capability, result in vane retaining lug failure. The present invention provides a method for the repair of such lugs, or for the reconfiguration thereof in the event of a design modification. The present invention further provides a method for the repair or reconfiguration of other parts or assemblies, where joining or addition of new material is necessary.
In the past, such repair or modification has been difficult due to the nature of the materials involved. Turbine blade and vane assemblies are frequently made from a cast single crystal material, for which welding is not a suitable repair technique due to microcracking or strain age cracking, and since local heat and working will cause a single crystal alloy to recrystallize, such a treatment would compromise the very properties for which the alloy was selected. The preferred material for repair of a cast part would logically be the same cast material, since the properties of the repair material would then be identical to those of the item to which the repair was made. Further, cast materials have critical superior properties, specifically creep resistance, which are important to the purpose and utilization of the parts fabricated therefrom. Unfortunately, this very property makes it impossible to utilize the parent material for a repair in which new material is joined to a cast material part, since the superior creep resistance actually inhibits the necessary material flow required to establish a quality bond. Attempts to build up a broken area by such techniques as plasma spraying additional material onto the broken area do not typically meet structural creep and fatigue property requirements at elevated temperatures.
Since many cast, hardenable nickel-base materials are not capable of welding, means were sought to achieve a solid state bond of a replacement blank to a cast assembly. It was found that cast replacement blanks could not effectively be forge joined to a cast assembly, due to high joining load requirements and the inability to achieve adequate deformation to avoid continuous interfacial carbide precipitation at the bond interface. Wrougt materials, on the other hand, are generally more suitable for forge joining, and a number of techniques are known for the bonding of wrought alloys. An exemplary development is the Gatorizing.RTM. isothermal forging method useful with high temperature alloys, as described in commonly owned U.S. Pat. No. 3,519,503, of Moore et al, the teachings of which are incorporated herein by reference. In addition, commonly owned U.S. Pat. No. 4,873,751, of Walker et al, teaches a fabrication or repair technique for integral bladed rotors, wherein the rotor and blades were forged from the same material. In addition, U.S. Pat. No. 4,883,216, of Patsfall, also teaches a method for the repair of a damaged part by bonding a replacement part to the stub of the damaged projection. However, prior to the present invention, no technique was available for the successful joining of a cast superalloy to wrought superalloy, such as would be necessary in the repair of the retaining lugs or feet of stator vane assemblies, or the joining of parts of differing materials to form an assembly having unique properties and capabilities, such as bonding cast blades to a wrought disk.